The present invention relates to the grain structure of workpieces formed from metals and metal alloys and, more particularly, relates to an apparatus and an associated method for producing a refined grain structure in a workpiece.
Structural assemblies, such as those in the aerospace industry, are often constructed by joining structural members together. During use, these structural assemblies can be subjected to a variety of environmental conditions, temperature variations, load variations, severe acoustic and vibration environments, all of which create mechanical and thermal stresses. The reliability and performance of the structural assemblies under these stresses depends in large part on the material properties of the constituent structural members and any weld joints between the members.
It is commonly recognized that the grain structure of structural members can have an adverse effect on the material properties of the structural members and any weld joints between the members. For example, the grain structure typically associated with conventional aluminum mill products can limit the formability, toughness, weldability, corrosion resistance and strength of structural members formed from these products. As an indication of formability, the typical elongation of AA 2195 aluminum alloys in the T8A3 condition along the longitudinal axis is approximately 11%. The typical elongation of AA 2219 aluminum alloys in the T87 condition along the longitudinal axis is approximately 10%. It is generally believed that the low formability of conventional aluminum mill products, especially in the AA 2195 aluminum alloys, is due to directionality of the grains and poor interlamninar strength. In addition, conventional aluminum mill products joined using common fusion welding techniques typically exhibit weld cracking in the heat affected zone, which can result in relatively weak weld joints. It is generally believed that the poor weldability of conventional aluminum mill products is a result of constitutional liquidation along the grain boundaries as the products are welded.
In addition too conventional aluminum wrought products, metal matrix composites have been implemented in the aerospace industry where high specific strength is required. Metal matrix composites are typically fabricated using powder metallurgy. Powder metallurgy products consist of fine metal powder and ceramic particles compressed together under controlled temperature and pressure (sintering) to produce a billet of material. The high expense associated with the production of fine metal powder and the sintering process makes these powder metallurgy billets less affordable.
In seeking to improve the material properties of structural members constructed of metals and metal alloys, it has been proposed to refine the grain size of the structural members through a process known as xe2x80x9cequal angle extrusion.xe2x80x9d As illustrated in FIG. 1, equal angle extrusion involves forcing a workpiece 10, using pneumatic or hydraulic pressure, through a die 12 have a 90xc2x0 bend. In theory, equal angle extrusion crushes the existing grain structure of the workpiece 10 such that the resulting material exiting the extrusion die 12 will exhibit a reduction in grain size. However, difficulties associated with large loads on the die 12 and cracking of the workpiece 10, can adversely affect the properties of the material existing the die. As a result, equal angle extrusion has not been used in large-scale production.
Thus, there remains a need for an apparatus for refining the grain structure of workpieces to thereby provide structural members having improved material properties, such as formability, weldability, toughness, corrosion resistance, and strength. The apparatus should be capable of operating on workpieces that are formed of a variety of metals and metal alloys and that have a variety of configurations. The apparatus also should be cost effective and should be scalable for use in large-scale production operations.
The present invention provides an apparatus and associated method for operating on a workpicce. According to one embodiment of the present invention, the apparatus comprises a die defining first and second apertures and an interior therebetween. The first aperture and the interior of the die are structured to receive the workpiece. The apparatus includes at least one rotatable pin extending at least partially into the interior of the die. In one embodiment, the die has first and second sides at least partially defining the interior and wherein the pin extends from the first side to the second side so as to traverse the interior of the die. The pin is structured to at least partially stir the workpiece as the workpiece moves through the interior of the die to thereby refine the grain structure of the workpiece. In one embodiment, the apparatus includes a ram structured to urge the workpiece through the interior of the die from the first aperture to the second aperture.
The interior of the die can be structured to shape the workpiece into a predetermined configuration, such as a square, a rectangle or a cylinder, to thereby cost effectively combine the operations of shaping the workpiece and refining the grain structure of the workpiece. In another embodiment, the interior of the die defines a chamber adjacent the second aperture that is structured to consolidate the workpiece. The rotatable pin can extend into the interior of the die between the first aperture and the chamber.
The apparatus can include a plurality of rotatable pins extending at least partially into the interior. Each of the pins is structured to stir the workpiece as the workpiece moves through the interior of the die. In another embodiment, the apparatus comprises a rotatable turret to which the plurality of pins are rotatably mounted. The turret is structured to individually rotate each of the pins in corresponding first directions and to collectively rotate the pins in a second direction. In one embodiment, the corresponding first directions are the same as the second direction. In another embodiment, the corresponding first directions are opposite to the second direction. In still another embodiment, the corresponding first directions comprise rotating at least two of the pins in opposite directions.
According to another embodiment of the present invention, the apparatus comprises at least one motor having a rotatable spindle. The apparatus includes a die defining first and second apertures and an interior extending therebetween, wherein the interior of the die is structured to form the workpiece. For example, in one embodiment, the interior of the die is structured to shape the workpiece into a predetermined configuration, such as a square, a rectangle or a cylinder. The apparatus includes at least one pin in rotatable communication with the spindle. The pin extends at least partially into the interior of the die and is structured to at least partially mix the workpiece as the workpiece moves through the interior to thereby refine the grain structure of the workpiece. In one embodiment, the die has first and second sides at least partially defining the interior and wherein the pin extends from the first side to the second side so as to traverse the interior of the die. The apparatus can include a ram structured to urge the workpiece through the interior of the die from the first aperture to the second aperture.
In another embodiment, the apparatus comprises a rotatable turret that is in rotatable communication with the spindle of the motor. The apparatus includes a plurality of pins each being in rotatable communication with the turret. Each of the pins extends from the turret at least partially into the interior of the die. The turret is structured to individually rotate each of the pins in corresponding first directions and to collectively rotate the pins in a second direction. The pins are structured to at least partially mix the workpiece as the workpiece moves through the interior of the die to thereby refine the grain structure of the workpiece. In one embodiment, the corresponding first directions are the same as the second direction. In another embodiment, the corresponding first directions are opposite to the second direction. In still another embodiment, the corresponding first directions comprise rotating at least two of the pins in opposite directions.
In another embodiment, the apparatus comprises a plurality of motors each having a rotatable spindle. The apparatus includes a plurality of pins each being in rotatable communication with a corresponding one of the spindles. Each of the pins extends at least partially into the interior of the die. The pins are structured to at least partially mix the workpiece as the workpiece moves through the interior of the die to thereby refine the grain structure of the workpiece.
The present invention also provides a method of operating on a workpiece. According to one embodiment, the method includes moving a workpiece through a die. Concurrently with the moving step, the workpiece is mixed with at least one rotating pin to thereby refine the grain structure of the workpiece. In one embodiment, the workpiece is heated concurrently with the mixing step. In another embodiment, the pin and/or die are heated concurrently with the mixing step. For example, the heating step can comprise heating the pin and/or die to a temperature between about 600xc2x0 F. and about 1000xc2x0 F. In another embodiment, the die is cooled concurrently with the mixing step. In yet another embodiment, the mixing step comprises individually rotating a plurality of pins in corresponding first directions and collectively rotating the plurality of pins in a second direction. In one embodiment, the corresponding first directions are the same as the second direction. In another embodiment, the corresponding first directions are opposite to the second direction. In still another embodiment, the corresponding first directions comprise rotating at least two of the pins in opposite directions.
The material properties of the workpiece can be further developed through selective beat treating. In one embodiment, the workpiece is solution heat treated prior to the moving step. In another embodiment, the workpiece is solution heat treated at a predetermined temperature schedule after the mixing step. In yet another embodiment, the workpiece is precipitation heat treated by aging at a second predetermined temperature schedule after the solution heat treating step.
The present invention also provides a preform for use in forming structural members and assemblies. According to one embodiment of the present invention, the preform comprises a body having predetermined dimensions and wherein the body comprises a refined grain structure having a cross-section comprising a curvilinear configuration. In one embodiment, the grain structure of the body comprises grains having equiaxed shape. In another embodiment, the grain structure comprises grains having a grain size of about 3 microns to about 5 microns. In another embodiment, the body is formed of aluminum, an aluminum alloy, titanium, a titanium alloy or a steel alloy. In still another embodiment, the body has an elongation of between about 15% and about 35%. In yet another embodiment, the body has an elongation of greater than about 20%.
Accordingly, there has been provided an apparatus and associated method for refining the grain structure of workpieces. The apparatus is capable of operating on workpieces that are formed of a variety of metals and metal alloys and that have a variety of configurations. In addition, the apparatus can be used in large-scale production to cost effectively combine operations, such as forming or shaping the workpieces, while at the same time refining the grain structure of the workpieces to thereby improve the material properties of the workpieces.